Bleaching of mechanical wood pulp



Patented Aug. 31, 1954 BLEACHING OF MECHANICAL WOOD PULP Robert H. Pete, Cincinnati, Ohio, assignor to E. I.

du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application June 26, 1951, Serial No. 233,701

8 Claims.

This invention relates to the bleaching of mechanical wood pulps such as those obtained by grinding wood.

Mechanical wood pulps are commonly and advantageously bleached by treatment with alkaline peroxide solutions. However, the efiectiveness of the bleaching treatment varies considerably with the wood species and also somewhat with the age of the pulp being bleached. Mechanical pulps from Eastern woods such as spruce, balsam, poplar and fir are the most amenable to bleaching. Those from tamarack, hemlock, jack pine and from the Western woods are usually darker and respond less favorably to known alkaline peroxide bleaching treatments.

One object of the invention is to provide an improved method of bleaching mechanical wood pulps with peroxide. A further object is to provide an improved method of bleaching mechanical wood pulps which respond relatively poorly to bleaching with peroxide by the usual treating procedures, which method involves a simple, inexpensive treatment of the pulp prior to bleaching it with alkaline peroxide. Still further objects will be apparent from the following description.

The above objects are accomplished in accordance with the invention by treating a mechanical wood pulp under non-alkaline conditions with a solution of a non-oxidizing calcium salt and subsequently bleaching the so-treated pulp with an alkaline peroxide solution. It has been discovered that the effectiveness of the peroxide bleach is substantially enhanced by the pretreatment with a solution of a non-oxidizing calcium salt in accordance with the present invention.

In a preferred method of practicing the invention, mechanical wood pulp is contacted at ordinary temperature and at apulp consistency of 5 to with an aqueous solution of calcium chloride at a pH of 3 to 6 and for a period of about 5 to 30 minutes. The calcium chloride solution used will contain an amount of CaClz equal to about 2 to 5% by weight, based on the dry weight of the pulp. The resulting pulp slurry is then dewatered, e. g., by filtration, centrifuging or the like, after which the pulp is bleached by means of an aqueous alkaline peroxide solution at ordinary or slightly elevated temperatures, e. g., by known peroxide bleaching procedures.

All percentages of reagents given herein are weight percentages based on the dry weight of the pulp, unless stated to be otherwise. The pulp consistencies given represent the percentlowing examples.

Example 1 Samples of ground hemlock wood pulp having a Hunter brightness value of 50.1 were pretreated with solutions of calcium chloride containing 2 and 5% CaClz by weight, at a 5% pulp consistency for 30 minutes at room temperature (77 to 86 F.). The pH of the mixtures of pulp and calcium chloride solutions during the treatments was within the range 3 to 6. The samples were then dewatered to 25% consistency on a Buchner funnel. A bleaching solution was then mixed with the dewatered pulp so as to reduce the consistency to 13%, and. the pulp was bleached at 100 F. In some of the bleaching trials, the bleach solution used (Solution A) was made by adding g. of sodium silicate solution (10% NazO, 25% S102 and 65% E20 by weight based on the Weight of the silicate solution), 22.5 g. H2SO4, 2.5 g. MgSOr'lHzO and 30 g. Nazoz per liter of water. In other trials the bleach solution used (Solution B) was made by adding 50 g. sodium silicate solution, 10 g. H2804, 2.5 g. MgSOa-7H2O and 20 g. Nazoz per liter of water. In the trials using Solution A, the active oxygen content of the solution used was equivalent to a weight of H202 equal to 1.3% of the dry weight of the pulp. In the trials using Solution B, the active oxygen content of the solution used was equivalent to a weight of H202 equal to 0.87% of the weight of the pulp.

For comparison purposes, samples of the same lot of hemlock pulp dewatered to 25% consistency were bleached in the same manner at 13% consistency except that the pretreatment with calcium chloride solution was omitted. The bleaching results obtained are tabulated below.

Percent Percent H202 each in Final Hunter Bright- Pretreat Mins. pH Brlghtness ment Initial Final Hess Gam 0. 87 0. O0 10. l 50. 6 0. 5 1. 3 0. 02 45 10. 1 54. 6 4. 5 0. 87 0. 04 10. 1 54. 9 4. 8 l. 3 0. 15 60 9. 9 59. 2 9. l 0. 87 0. 10 50 10. 0 56. l 6. U 1. 3 0. 16 9. 6 60. 9 10. 8

All brightness measurements were made using a Hunter Reflectometer on air dried handsheets prepared as described in TAPPI procedure N0. T. 205.

Example 2 Mechanical wood pulps obtained by grinding wood of the types indicated in the table below were pretreated with calcium chloride and then bleached in the general manner described in Example 1. Comparative experiments were also run in which the calcium chloride pretreatment was hypochlorite for enhancing the bleaching obtained with a subsequent peroxide treatment. This is surprising since the advantages heretofore accre dited to pretreatments with calcium hypochlorite have generally been considered to be due to the action of active chlorine on the pulp. Non-oxidizing calcium salts such as calcium chloride, calcium nitrate, calcium acetate and the like, of course contain no active chlorine or an equivalent conomitted. In each experiment, bleaching was ter- 10 Stituent in View of which it been minated when the active oxygen content, of the expected such Salts Would be ineffective as pulp slurry fell to a value equal to about 0.17% p at i agents. H202 based on the dry weight of the pulp. The Indications are that the improvement in tabulation below shows the results obtained. bleaching resulting from the present pretreat- Percent Percent H201 5, g g: Mm. g 5552i? 3533. rep p treatment Initial Final ness Gam Spruce none 1. 3 90 60. 6 8. Do 5 1.3 0.19 300 63.2 11.1 Old Balsam none 0. 87 0.18 60 9. 4 62. 1 10.0 Do 5 0.87 0.18 180 9.1 66.1 14. 0 New Balsam... none 0. 87 0. 13 60 9. 6 66. 6 6. l D 5 0. s7 0. 17 180 9. 2 68. 7 8.2 none 0. 87 o. 17 60 9. 4 70. 1 9. 0 5 0. s7 0. 22 180 9. 1 13. 2 12. 1 none 0. s1 0. 09 so 9. 4 55. a 7. o 5 0.87 0. 18 120 9. 2 60. 4 11.9

Example 3 ment is due to the presence of the calcium ion in 30 the pretreating liquor rather than to the presence A sample of a groundwoo'd pulp containing equal parts by weight of Western spruce and Western hemlock was treated at a 3% pulp consistency for minutes at 80 F. with a neutral calcium chloride solution containing 1.42% by weight CaCl2 based on the dry Weight of the pulp. The pH of the resulting slurry after 1 and 10 minutes of mixing was 5.2 and 5.17, respectively. After the treatment, the slurry was dewatered by centrifuging. The dewatered pulp was then bleached with an alkaline peroxide solution at a pulp consistency of 12.3% and a temperature of 110 F. The bleach solution used contained 0.025 g. Epsom salt, 5 g. sodium silicate solution, 3 g. Na2O2 and 2.25 g. H2804 per hundred grams of dry pulp treated. After 2 hours bleaching time the pH of the slurry was 9.2, the active oxygen content of the slurry corresponded to 0.08% H202 based on the weight of the dry pulp, and the pulp brightness was 63.7, representing a brightness gain of 9.6 points. After 4 hours bleaching, the pH was 9.2, the H202 equivalent of the active oxygen present was 0.03% and the pulp brightness was 63.3, representing a brightness gain of 9.2 points.

In a comparison experiment, a sample of the same pulp mixture was bleached under the same conditions described above except that a neutral solution containing a molecularly equivalent amount of calcium hypochlorite was used in the pretreatment in place ofcalcium chloride. The pH of the slurry of pulp in the hypochlorite solution after 1 and 10 minutes of mixing was 5.8 and 5.0, respectively. After 2 hours bleaching time with the alkaline peroxide solution, the pH of the pulp slurry was 9.1 and the H202 equivalent of the active oxygen present was 0.14%. The pulp brightness at this stage was 63.1, representing a brightness gain of 9.0 points. After 4 hours bleaching time, the pH was 8.9, the H202 equivalent of the active oxygen present was 0.07% and the pulp brightness 64.1, representing a brightness gain of 10.0 points.

The results of Example 3 demonstrate that a pretreatment with calcium chloride is substantialof chloride, nitrate or other ion introduced along with the calcium ion. Thus, it has been found that a pretreatment with sodium chloride instead of calcium chloride is entirely ineffective for the present purpose.

Improvement in bleaching results can be obtained when efiecting the pretreatment with solutions of non-oxidizing calcium salts under a wide variety of conditions. Ordinarily, however, the amount of calcium salt used should be at least about 0.1% based on the dry weight of the pulp. Smaller amounts are not significantly effective. Amounts greater than about 6% can be used but result in no added advantages and increase costs. The preferred amounts range from about 2 to about 5%. As a general proposition, when eifecting the pretreatment at low pulp consistencies where the concentrations of both pulp and salt will be dilute, somewhat larger amounts of the salt are desired than when pretreating at higher consistencies. While the pretreatment may be carried out at any desired pulp consistency, consistencies ranging from about 5 to and amounts of calcium salt equal to 5 to 2% of the dry pulp weight have been found to be most satisfactory and are preferred.

The time of the pretreatment is not critical so long as sufiicient time is provided for thoroughly mixing the treating solution with the pulp. When dilute salt solutions are employed the contact time may advantageously be slightly longer than when employing more concentrated solutions. In general, contact times of 5 to minutes are entirely satisfactory. The pretreatment can be effected at ordinary or higher temperatures, ordinary temperatures being most convenient and preferred. The temperature during the pretreatment is not a critical factor and temperatures much higher than ordinary temperatures can be employed effectively.

The salt-pulp-water system should be nonalkaline, i. e. have a pH not higher than 7, during the pretreatment. The generally effective pH range is 1 to 7 but acidities in the pH range 3 to 6 1y as effective as is a pretreatment with calcium are most effective. Pretreatment under alkaline conditions, i. e. at a pH substantially above 7, is substantially inefiective while an acidity corresponding to a pH lower than about 1 is generally impractical. During the treatment the calcium ion appears to migrate into the pulp, possibly by ion-exchange reaction. It is advisable to agitate the mixture of pulp and treating solution so that the pulp becomes uniformly wetted by the solution. Since the calcium ion is taken up by the pulp, the pulp can be washed if desired following the pretreatment and prior to bleaching with peroxide without eliminating the advantages resulting from pretreatment. However, such intermediate washing is not necessary.

The subsequent peroxide bleaching treatment can be carried out in any desired manner employing well-known bleaching procedures. Ordinarily, the bleaching liquor will have a pH within the range 9 to 12, preferably 10 to 11.5, and the bleaching treatment will be carried out at ordinary or slightly higher temperatures, e. g. 80 to 150 F., preferably 100 to 130 F. Pulp consistency is not critical during the bleaching stage and bleaching can be effected at high, medium or low consistencies. Generally, bleaching is somewhat more effective at the higher consistencies. The bleach liquor can be prepared employing any active oxygen yielding peroxygen compound such as hydrogen peroxide, alkali metal peroxides or persalts such as sodium perborate, percarbonate, perphosphate and the like. Bleach liquors containing active oxygen in amounts equivalent to about 0.5 to 1.5% H202 based on the dry weight of the pulp are most commonly used in bleaching procedures of this type and are generally satisfactory.

I claim:

1. The method of bleaching a mechanical wood pulp comprising treating the pulp under nonalkaline conditions with a solution of a non-oxidizing calcium salt, and thereafter bleaching the pulp with an alkaline peroxide solution.

2. The method of bleaching a mechanical wood pulp comprising treating said pulp at a pH of 1 to 7 with an aqueous solution of a non-oxidizing calcium salt, said solution containing said salt in an amount equal to 0.1 to 6% by weight of the dry weight of the pulp, and thereafter bleaching said pulp with an alkaline peroxide solution.

3. The method of claim 2 wherein the pulp is first treated at a pH of 3 to 6 with a solution containing a calcium salt in an amount equivalent to 2 to 5% by weight based on the weight of the pulp.

4. The method of claim 2 wherein the salt used is calcium chloride.

5. The method of claim 3 wherein the salt used is calcium chloride.

6. The method of claim 1 employing an alkaline peroxide solution containing sodium silicate.

7. The method of claim 2 employing an alkaline peroxide solution containing sodium silicate.

8. The method of claim 3 employing an alkaline peroxide solution containing sodium silicate.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 354,477 Just Dec. 14, 1886 1,991,824 Snyder Feb. 19, 1935 2,199,376 Reichert Apr. 30, 1940 2,359,891 Brallier et al. Oct. 10, 1944 2,398,420 Freeman Apr. 16, 1946 2,465,738 McEwen Mar. 29, 1949 FOREIGN PATENTS Number Country Date 917,993 France Jan. 27, 1947 

1. THE METHOD OF BLEACHING A MECHANICAL WOOD PULP COMPRISING TREATING THE PULP UNDER NONALKALINE CONDITIONS WITH A SOLUTION OF A NON-OXIDIZING CALCIUM SALT, AND THEREAFTER BLEACHING THE PULP WITH AN ALKALINE PEROXIDE SOLUTION. 